I've had this tree for about 7 months. I watered it according to the nursery instructions...water until it runs into the tray then wait until it is dry for about 6 inches on the top. I also fertilized with winter mix (from the nursery) until Spring, then with growth mix (also from the nursery). Until recently there was good growth and quite a few flowers (although as soon as they started to become lemons they dropped off). In the past month plus it's started dropping leaves and when I used a moisture probe it showed very moist even though it felt dry to the touch more than six inches down. I tried mixing some orchid mulch into the area around the roots and haven't watered for quite a long time. Nothing works so far and I think there may be mold mites infesting it. I'm really discouraged. I've done everything I was supposed to do (according to the nursery) and even some suggestions from this site. I'm afraid I'm going to lose the tree.
The most common cause of citrus dropping leaves is root rot due to over-watering, even temporarily. The pic does not show how your pot is mounted, but ideally you will have the pot up on pebbles so that any excess water gets immediately drained away from the pot. Sitting in water would be very bad for lemon. From your description you have been careful to avoid this.
Leaf drop can also happen when there is a sudden temperature change. Note how the branch leaning away from the window has leaves. From the heater radiator visible against the wall you have central heating which indicates that the window can experience sudden cold. In really cold weather and sudden changes pull the pot away from the window towards the centre of the room where the temperature changes will be less dramatic. A brightly lit window due to direct sun does not necessarily imply a warm window.
PS: check the window/door for leaks. Everything looks impressively clean and new but a cold breeze that filters in rapidly would be a shock close to the window even if not felt in the centre of the room. Do a smoke test rather like that performed while doing an energy audit.